The present invention relates to industrial process control and monitoring systems. More specifically, the present invention relates to wireless field devices used in such systems.
In industrial settings, control and monitoring systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process and coupled to control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system, including all devices used in the measurement, control and monitoring of industrial processes.
Field devices are used by the process control and measurement industry for a variety of purposes. Usually, such devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and are able to withstand climatological extremes of temperature, humidity, vibration, mechanical shock, et cetera. These devices also can typically operate on relatively low power. For example, field devices are currently available that receive all of their operating power from a known 4-20 mA loop.
Some field devices include a transducer. A transducer is understood to mean either a device that generates an electrical output based on a physical input or that generates a physical output based on an electrical input signal. Typically, a transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and others.
Typically, each field device also includes communication circuitry that is used for communicating with a process control room, or other circuitry, over a process control loop. In some installations, the process control loop is also used to deliver a regulated current and/or voltage to the field device for powering the field device.
Traditionally, analog field devices have been connected to the control room by two-wire process control current loops, with each device being connected to the control room by a single two-wire control loop. Some analog field devices transmit a signal to the control room by modulating the current running through the current loop to a current that is proportional to a sensed process variable. Other analog field devices can perform an action under the control of the control room by controlling the magnitude of the current through the loop. In addition to, or in the alternative, the process control loop can carry digital signals used for communication with field devices. Digital communication allows a much larger degree of communication than analog communication. Moreover, digital devices also do not require separate wiring for each field device. Field devices that communicate digitally can respond to and communicate selectively with the control room and/or other field devices. Further, such devices can provide additional signaling such as diagnostics and/or alarms.
In some installations, wireless technology is used to communicate with field devices. A wireless configuration simplifies field device wiring and setup. Various wireless communication techniques can be employed including a mesh network type configuration. One example wireless communication protocol is Wireless HART® in accordance with the IEC 62591 standard. However, as there are no physical electrical connections to the wireless field device, the wireless field devices are typically required to operate using relatively little power. Low powered components and modules of such devices can be damaged if they are improperly connected to a power source, or connected to a power source having an output voltage which exceeds the limits of the component or module.